Pressure-fluid-operated, especially compressed-air-operated control cylinders--compressed-air-cylinders--are often used to provide linear motions. These operating cylinders are almost exclusively installed to work between two fixed stops, usually the two end positions of the piston. In case the piston is to enter the position between the two end positions a brake means or some other type of mechanical stop is required. The reason for this is that the pressure fluid--the air -is compressible and therefore cannot keep the piston in a distinct fixed position if it is actuated by an outer force or load that may vary from time to time or in time.
However, when a brake means or a mechanical stop is used the piston will both start and stop with a heavy jerk due to the fact that the piston is loaded by a sufficiently great fluid pressure in order that it might not "sink through" at the start and the piston cannot be stopped in correct position merely by reducing the pressure.
The problem is well-known, and for said reason cylinders are mostly used which operate with incompressible fluid hydraulics when it is desired to position the piston in definite, arbitrarily selected positions. The supply of pressure fluid--oil--is blocked by the aid of valves, the piston being locked in the intended position. However, hydraulic systems have disadvantages in the form of great installation costs, expensive operation and maintenance. Moreover, there is also a risk that leakage of the hydraulic oil can involve inconvenience.